Diluters are necessary in the process of exhaust sampling, before the exhaust is led to instrumentation for analysis. In particular, diluters reduce the temperature and the humidity of the exhaust gas which, otherwise, might not be tolerated by the analytical instrumentation. In addition they reduce the concentration of pollutants and equilibrate the sample before analysis.
Exhaust gas dilution may be achieved with the use of full flow diluters, wherein all of the exhaust gas is led to a dilution tunnel where it mixes with ambient filtered air before analysis. Such dilution systems include the constant volume sampling technique, as described in legislation 91/441/EEC of the European Commission (Official Journal L 242, Aug. 30, 1991 P. 0001-0106). Constant volume, full flow diluters lead to variable dilution ratios over transient tests, as the mixing proportions of exhaust gas and dilution air change, depending on the momentary flowrate of exhaust gas. The variable dilution ratio is a significant shortcoming when it comes to the accurate characterization of emissions, in particular of particulate matter emissions, because it introduces variable degrees of particle nucleation and condensation, depending on the saturation ratio of volatile and semi-volatile species following dilution.
To overcome the issue of variable dilution ratio, partial flow sampling systems have become widespread, where only a portion of the exhaust gas is sampled, measured and conditioned with a measured quantity of dilution air, to provide a fixed dilution ratio. The dilution ratios typically achieved by such systems range from 10:1 to 1000:1. This wide range is required to reduce particle levels from different sources, such as conventional diesel engines, diesel with particle filters within the measuring range of the available instrumentation. Such partial flow systems are described for example in the patent applications US2003/0232449A1, JP2004205253A2004.T.22. These devices mainly target at sampling exhaust downstream of aftertreatment devices because the dilution ratio provided is very sensitive to the exhaust gas pressure variation. Upstream of aftertreatment devices and, in particular, of diesel particle filters, the exhaust backpressure may vary between a few millibars to several hundreds of millibars above ambient, resulting in significant variation of the dilution ratio.
The device presented in patent CH693491 addresses the issue of providing a constant dilution ratio with small sensitivity to upstream exhaust gas conditions but introduces a number of moving parts, which are not desirable for the dilution of the hot, humid and particle laden exhaust gas.